I did actually take this up with the company concerned (at least the company mentioned on the invitation I received). They have replied and said they were going to take it further.
But unless they can find the person (and who knows what country he is operating from), I don't suppose there is very much they, or anyone can do. But Upwork should make it their business to net this sort of scamster before the proposal gets posted. (By the way the job invitation I had is still up and I have lost count of the times I have flagged it) It is not good for their reputation.
I'm glad I found this thread via Google as I was sent a text from a job poster telling me to check my spam for an email from them which told me to go on Yahoo messenger and add them for a chat. The job poster from upwork who was interviewing me on Yahoo Messenger made me become suspicious as to why we were speaking on Yahoo & so I started researching in between messages.